1. Field of the Invention
The present invention relates to a magnetic recording medium and a method for producing the same. Particularly, it relates to a perpendicular magnetic recording medium having a magnetic recording pattern formed therein and a method for producing the perpendicular magnetic recording medium.
2. Description of the Background Art
A stationary magnetic storage device (hard disk drive) is used as one of information recording devices for supporting recent sophisticated information society. Improvement in recording density is required of a magnetic recording medium used in the magnetic storage device with the increase in quantity of information. To achieve high recording density, a unit in which inversion of magnetization occurs must be small-sized. To this end, it is important that magnetic grain size is reduced and the unit of inversion of magnetization is separated and partitioned clearly to reduce magnetic interaction between adjacent recording units.
As a technique for achieving higher-density magnetic recording, a perpendicular magnetic recording method has been recently used in place of a longitudinal magnetic recording method. As a magnetic recording layer material for use in a perpendicular magnetic recording medium, a CoCr alloy crystalline film having a hexagonal close-packed structure (hcp structure) is discussed chiefly at present. Perpendicular magnetic recording is performed while crystal orientation is controlled so that a c axis of the hcp structure is perpendicular to the film surface (i.e. a c surface is parallel to the film surface).
Approaches to refinement of crystal grains forming the CoCr alloy crystalline film, reduction of the grain size distribution, reduction of magnetic interaction between grains, etc. have been proposed to cope with further increase in density of the magnetic recording medium in future.
As a magnetic layer structure control method for higher magnetic recording density, there is a method using a magnetic layer having a structure in which the periphery of a magnetic crystal grain is surrounded by a non-magnetic non-metal substance such as oxide or nitride, generally called granular magnetic layer. Low noise characteristic is obtained in the granular magnetic film because the grain boundary phase of the non-magnetic non-metal substance separates magnetic grains physically so that magnetic interaction between magnetic grains is lowered to thereby suppress zigzag domain walls from being formed in a recording bit transition region.
Generally, a perpendicular magnetic recording medium using Ru as an underlying layer and using a CoPtCrO alloy having a granular structure as a magnetic layer has been proposed. As the thickness of the Ru layer which is the underlying layer located under the granular magnetic layer is increased, c axis orientation is improved. Excellent magnetic characteristic and electromagnetic transducing characteristic are obtained with the improvement of c axis orientation.
It has been reported that when RF sputtering film formation is performed by using a CoNiPt target containing oxide such as SiO2, a granular recording film having a structure in which respective magnetic crystal grains are surrounded by non-magnetic oxide so as to be separated individually can be formed to thereby achieve low noise characteristic (e.g. see U.S. Pat. No. 5,679,473, and so on). It is conceived that low noise characteristic is obtained in such a granular magnetic film because the grain boundary phase of the non-magnetic non-metal substance separates magnetic grains physically so that magnetic interaction between the magnetic grains is lowered to thereby suppress zigzag domain walls from being formed in a recording bit transition region.
When a crystal orientation control layer having the same crystal structure, that is, the same hcp structure as that of ferromagnetic crystal grains of the magnetic layer is provided, Co grains of the magnetic layer is grown on the crystal orientation control layer corresponding to a crystalline substance (crystal grains) of the crystal orientation control layer and that oxide is precipitated and grown in the magnetic layer corresponding to a crystal grain boundary porous region or amorphous region of the crystal orientation control layer. In other words, there is a proposal that crystal grains of the magnetic layer are grown epitaxially on crystal grains of the crystal orientation control layer, so that crystal orientation of the crystal orientation control layer is taken over to the magnetic layer to thereby make it possible to control the crystal orientation of the magnetic layer, and that an amorphous phase crystal gain boundary interposed in the periphery of crystal grains forming the magnetic layer is formed to thereby make it possible to control the crystal state of the magnetic layer having a granular structure (e.g. see JP-A-2003-123239, JP-A-2003-242623, and so on).
It has been reported that when a magnetic layer of a perpendicular magnetic recording medium includes a first magnetic layer of a granular structure and a second magnetic layer of a non-granular structure, good electromagnetic transducing characteristic and high durability can be guaranteed (e.g. see JP-A-2007-103008, and so on).
In order to improve easiness of recording without spoiling thermal stability, there has been proposed a layer structure including a first magnetic recording layer and a second magnetic recording layer which are ferromagnetically coupled to each other while a coupling layer is interposed therebetween. It has been reported that at least one of the first magnetic recording layer and the second magnetic recording layer preferably has a granular structure (e.g. see JP-A-2006-48900, and so on).
In such a granular perpendicular magnetic recording medium, relatively good magnetic characteristic and electromagnetic transducing characteristic are obtained. The granular perpendicular magnetic recording medium proposed heretofore was however a continuous film in plan view, that is, a so-called solid film. To achieve higher recording density, it is necessary to prevent writing from being blurred in adjacent tracks, reduce the formation of zigzag domain walls due to grains arranged at random, reduce the influence of thermal fluctuation due to reduction in crystal grain size, and reduce magnetic interaction between magnetic grains as much as possible.
Therefore, a discrete track medium has been proposed. The unit in which magnetization is inverted is partitioned clearly, that is, a magnetic substance row in which intervals of tracks are magnetically completely separated is formed to obtain an adjacent track boundary artificially. Writing can be prevented from being blurred in the adjacent tracks, and zigzag domain walls can be prevented from being formed.
Further, a patterned medium has attracted attention. As the patterned medium, there has been proposed a patterned medium in which dots separated into single domains made uniform artificially in shape and size are arranged as an array so that each single magnetic material dot is used as a single recording bit for performing recording and reproducing (e.g. see JP-A-10-233015, and so on).
Although various techniques known heretofore can be used as a method of forming a structure in which a magnetic material of a patterned medium is separated, improvement is required because each of the techniques has advantages and disadvantages. For example, photolithography has an advantage in terms of throughput because of batch exposure but comes with such difficulty that a large area of the magnetic recording medium is exposed to light in batch with a fine pattern of dozen nm. Although an electron beam lithography method or a converged ion beam method can form a fine pattern of dozen nm because an electron beam or a converged ion beam is irradiated while following along the pattern, the electron beam lithography method or the converged ion beam method is not realistic in terms of processing cost based on processing time because few days are required for processing all the large area of the magnetic recording medium.
Several methods using self-assembling have been proposed as a method for solving this disadvantage. For example, there has been proposed a method of producing a magnetic recording medium, in which fine grains with a diameter in a range of from the order of nanometers to the order of micrometers are two-dimensionally arranged on a substrate and patterning is performed with the fine grains as a mask to thereby form magnetic fine grains isolated on the substrate (e.g. see JP-A-10-320772, and so on).
A pattern forming method using a self-assembling phase separation structure of a block copolymer has been proposed (e.g. see P. Mansky et al, Appl. Phys. Lett., vol. 68, p. 2586, M. Park et al, Science, vol. 276, p. 1401, and so on). The method using a block copolymer can form a regularly arranged pattern by a very simple process of dissolving the block copolymer in a suitable solvent and applying the resulting solvent on a piece to be processed. Generally, the phase separation structure of the block copolymer is self-assembled as a honeycomb hexagonal close-packed lattice.
A magnetic recording medium which uses a self-assembling arrangement structure of anodized alumina pores so that the alumina pores are filled with magnetic metal has been also proposed (e.g. see JP-A-2002-175621, and so on). The magnetic recording medium has an underlying electrode layer and anodized alumina pores in this order on a substrate. The anodized alumina pores are formed so that a large number of alumina pores are arranged orderly. The alumina pores are filled with ferromagnetic metal to thereby form a ferromagnetic layer. Incidentally, the anodized alumina pores are generally self-assembled in the form of a honeycomb hexagonal close-packed lattice.
According to the arrangement of fine grains, the self-assembling of block copolymer and the self-assembling of anodized alumina, a fine array can be formed in a large area at low cost. Although the arrangement based on this method is provided as a two-dimensional arrangement which is well-ordered at a relatively short distance among dozen fine grains, the arrangement is disordered at a long distance and exhibits an aspect of a polycrystalline substance so that a large number of defective places occur when the magnetic recording medium is viewed as a whole.
Several methods have been proposed to solve this problem to thereby keep the whole magnetic recording medium well-ordered. For example, there has been proposed a method in which: a concave-convex line is formed on a substrate; fine grains are arranged as a patterned single layer on the concave-convex line; the arrangement pattern of the fine grains is transferred onto a stamper-forming material to thereby form a stamper; and a start point for forming nano-holes is formed on a metal substrate by using the stamper; and a nano-hole forming process is applied to the metal substrate (e.g. see JP-A-2006-346820, and so on). Or as for a method using self-assembling of a block copolymer, there has been proposed a recording medium having a structure in which: cells shaped like parallelograms surrounded by separation regions containing substantially parallel straight lines along a track direction and substantially parallel straight lines intersecting the first-mentioned straight lines at an angle of 60° or 120° are formed on a disk substrate; and a granular recording material is arranged to assemble a regular lattice in each of the cells (e.g. see JP-A-2002-334414, and so on).
As described above, there have been proposed various methods in which a recording unit for performing magnetic recording is made from a fine pattern to improve recording density. The methods of simply making the recording unit fine are however not enough to effectively perform an actual read/write operation used in a hard disk drive.
FIG. 1 is a schematic view showing a locus of movement of a magnetic recording head (hereinafter also referred to as head simply) when information is recorded/read on/from a magnetic recording medium by the magnetic recording head. To scan a recording portion of a disk-like magnetic recording medium 10 having a hole of an inner circumference 20 in its central portion and an outer circumference 24, a magnetic recording head 11 fixed to a head arm 14 makes a seeking operation between the inner circumferential side and the outer circumferential side of the doughnut-shaped magnetic recording medium by using a rotation center 12 of the head as a pivot to thereby form a locus 15 of the head. In a hard disk drive, magnetic recording is performed along a circumferential direction which is a track direction of the magnetic recording medium. Accordingly, a read/write element of the head (hereinafter referred to as “head RW element” for short) and a track intersect each other at an angle of intersection of the track direction and the locus of the head. An angle between the track width direction of the magnetic recording medium and the track width direction of the magnetic recording head is called skew angle.
FIG. 2 is a schematic enlarged view of an upper half of the magnetic recording medium showing a state where the skew angle α varies according to the radial track position. For example, on an intermediate circumference 22 in the radial direction of the magnetic recording medium, the radial direction (track width direction) of the magnetic recording medium and the track width direction of the head RW element coincide with each other, so that the skew angle is 0°. However, at other points, the radial direction of the magnetic recording medium and the direction of the head do not coincide with each other. On an inner circumference 21 in the radial direction of the magnetic recording medium, the skew angle is α°. On the outer circumference 23 in the radial direction of the magnetic recording medium, the skew angle is not 0° likewise. Generally, in a hard disk drive, the skew angle is allowed to be up to about ±15°.
In the background-art magnetic recording medium in which a magnetic recording layer is formed uniformly on the whole surface of the magnetic recording medium, the magnetic recording layer is present in the position of the head RW element even if there was the skew angle. The magnetic recording layer may be however absent in the position of the head RW element when the magnetic recording layer is patterned to be made from magnetic dots.
FIGS. 3 and 4 are schematic enlarged views showing a part of an upper surface of the magnetic recording layer of the magnetic recording medium for explaining a state where the positional relation between the head RW element and each patterned magnetic dot varies according to the skew angle. The magnetic recording layer is made from magnetic dots 30 which are arranged in a non-magnetic substance 31 to have such a predetermined pattern that one magnetic dot 30 is disposed in each rectangle formed from a track width 34 and a bit width 35. On the assumption that anodized alumina is self-assembled into pores shaped like a honeycomb hexagonal close-packed lattice, there is shown the case where six magnetic dots adjacent to a certain magnetic dot are substantially shaped like a hexagon 36. Arrows in FIGS. 3 and 4 express a track direction 32 of the magnetic recording medium and a radial direction 33 of the magnetic recording medium, respectively. A position where the head RW element is projected onto the magnetic recording medium is shown as a projection 37 of the head RW element.
Each of FIGS. 3 and 4 shows the case where the magnetic dots 30 are lined up and arranged regularly along the radial direction of the magnetic recording medium. FIG. 3 shows the case where the skew angle is 0°. FIG. 4 shows the case where the skew angle becomes large. In the case of FIG. 3, it is possible to read/write information without affecting other bits because one magnetic dot 30 corresponds to each projection 37 of the head RW element. When the skew angle becomes large, the projection 37 of the head RW element however comes out of the rectangle formed from the track width 34 and the bit width 35, as shown in FIG. 4. Consequently, it is known that it is impossible to read/write information accurately because the head RW element interferes with magnetic dots of adjacent bits.
A method of changing the arrangement of magnetic dots in accordance with the skew angle has been proposed (e.g. see JP-A-2006-73137) in order to avoid interference with adjacent bits even when the skew angle changes. In the proposed method, magnetic dots are made of nano-holes each filled with a magnetic material. To form the nano-holes, a line groove is formed and a row of nano-holes is formed in the line groove to thereby arrange magnetic dots. The direction of the line groove is changed according to the radial position of the magnetic recording medium to thereby arrange magnetic dots so that the direction of arrangement of the magnetic dots is substantially the same as the track width direction of the RW element of the magnetic recording head. According to the method, the same micro-fabrication level as that of the adjacent bit width is required as the minimum width of patterning. It is however considerably difficult to perform the same micro-fabrication as that of the bit width because the bit width is very fine.
It is preferable that one recording bit is made from one magnetic dot in order to reduce magnetic interaction between magnetic dots. When recording density is intended to be increased, however, it is necessary to reduce the bit size which is the smallest unit of information recording and it is therefore necessary to reduce the size of the magnetic dot. Accordingly, a higher-grade micro-fabrication technique is required for arranging such a small magnetic dot in a desired position.